Treatment of petroleum cokes to inhibit coke puffing

ABSTRACT

A process for treating high sulfur petroleum coke to inhibit puffing is disclosed wherein particles of the petroleum coke are contacted, in the absence of a binder, with a compound containing an alkali or alkaline earth metal selected from the group consisting of sodium, potassium, calcium and magnesium, at an elevated temperature above that at which the alkali or alkaline earth metal compound begins to react with carbon, but below the temperature at which the coke particles would begin to puff in the absence of the compound. The coke particles are maintained at the elevated temperature for a sufficient period of time to permit the reaction to proceed and allow products of reaction to penetrate into the particles and form an alkali-or alkaline-earth-metal-containing deposit throughout the mass of the particles; and then cooling the so-treated coke particles.

The present invention relates to carbon and graphite articles,particularly electric furnace electrodes, and to a process for producingsuch electrodes of improved quality using high sulfur petroleum cokes.More particularly, the invention relates to a process for treatingcalcined petroleum cokes with a puffing inhibitor prior to incorporatingthe coke into a carbonaceous mix. In an important aspect, the inventionrelates to a carbonaceous filler or aggregate containing discreteparticles of a calcined petroleum coke having a high sulfur content andhaving a puffing inhibiting agent distributed throughout the mass of theparticles, the inhibiting agent serving to reduce or eliminate cokepuffing during manufacture and use of graphite and carbon articles.

BACKGROUND OF THE INVENTION

It is common practice in the production of carbon and graphite electricfurnace electrodes to employ a calcined petroleum coke (i.e., rawpetroleum coke that has been heated to temperatures above about 1200°C.) as the filler or aggregate material and to mix this filler oraggregate with a carbonaceous binder such as pitch. The mixture isformed into the shape of the electrode, either by molding or extrusion,and is then baked at an elevated temperature sufficient to carbonize thebinder (e.g. about 800° C.). In those cases where a graphitizedelectrode is required, the baked electrode is further heated totemperatures of at least about 2800° C.

Petroleum coke particles have a tendency to "puff", that is, to expandand even to split when heated to temperatures above about 1500° C., ifthey contain more than about 0.3% by weight sulfur. Electrodes made fromsuch cokes lose density and strength and sometimes split lengthwise whenheated to these high temperatures. As indicated, graphite electrodes arenormally heated to at least 2800° C. during their manufacturing process.Carbon electrodes, which are not graphitized during the manufacturingprocess, reach temperatures between about 2000° C. and 2500° C. duringtheir use in silicon or phosphorus furnaces.

Puffing is associated with the release of sulfur from its bond withcarbon inside the coke particles. If the sulfur containing vapors cannotescape from the particles or from the electrode fast enough, they createinternal pressure which, in turn, increases the volume of the particlesand may cause the electrode to split.

The conventional remedy for puffing has been to add an inhibitor such asiron oxide or other metal compound to the coke-pitch mixture before theelectrodes have been formed. It has been shown, for example, that about2 weight percent iron oxide can be effective to reduce coke puffing.Some cokes that have a higher tendency to puff or start puffing at alower temperature cannot be adequately controlled by iron oxide.

Various attempts have been made to provide other improved puffinginhibition methods which overcomes the above and other disadvantages ofthe prior art. For example, in U.S. Pat. No. 2,814,076 issued to J. W.Gartland on Nov. 26, 1957, there is disclosed an improved method ofproducing graphite articles such as electric furnace electrodes whereinan alkali metal compound from group I of the Periodic table, notablysodium carbonate, is employed as a puffing inhibitor. The sodiumcarbonate may be added to the article by impregnating the article afterbaking with a solution of the sodium carbonate or by adding the puffinginhibitor directly to the coke-pitch mix. Although adding sodiumcarbonate to the coke-pitch mix is more convenient than adding it to thebaked article, this method produces a finished electrode of inferiorquality, i.e., lower density and lower strength.

Another problem encountered when the puffing inhibitor is added directlyto the coke-pitch mix is that sodium carbonate reacts with acidicextrusion aids which may be employed in the mix. Unfortunately, thisreaction often causes extrusion problems leading to poor structure ofthe electrode.

Another approach to solving the problem of coke puffing in theproduction of carbon and graphite electrodes is disclosed in U.S. Pat.No. 3,506,745 issued to L. H. Juel et al. on Apr. 14, 1970. In thisapproach, high sulfur petroleum coke particles are treated prior totheir incorporation in a carbonaceous mix by contacting the cokeparticles with a puffing inhibitor and heating the particles in asubstantially non-oxidizing atmosphere to temperatures above about 1400°C., and also above that at which the coke begins to puff in the absenceof the puffing inhibitor and preferably above 2000° C. The puffinginhibitor may be introduced by dusting fine powders of the inhibitoronto the granular petroleum coke or an aqueous slurry containing theinhibitor may be prepared and sprayed onto the coke before heating thecoke particles to puffing temperatures. The coke particles are thencooled to about ambient temperatures and blended with a pitch binder toform a conventional carbonaceous mix. The puffing inhibitor combineswith the sulfur and is volatilized when the coke is heated to puffingtemperatures and above. The problem with this approach is that theprocess requires heating the coke particles to temperatures that aresignificantly higher than those ordinarily employed during the usualcalcining process. Consequently, this treatment can only be carried outwith a process which is different from ordinary calcining practices,consuming more energy and requiring more expensive equipment.

SUMMAY OF THE INVENTION

The present invention is directed to an improved process for treatinghigh sulfur petroleum coke with a puffing inhibitor prior toincorporating the coke into a carbonaceous mix. In the broadest sense,the improved process comprises contacting the particles of the highsulfur petroleum coke with a compound containing an alkali or alkalineearth metal selected from the group consisting of sodium, potassium,calcium and magnesium, at an elevated temperature above that at whichthe alkali or alkaline earth metal compound begins to react with carbon,but below the temperature at which the coke particles would begin topuff in the absence of the compound; maintaining the coke particles atthe elevated temperature for a sufficient period to time to permit thereaction to proceed and allow products of reaction to penetrate theparticles and form an alkali or alkaline earth metal containing depositthroughout the mass of the particles; and then cooling the so-treatedcoke particles.

The process of the present invention is preferably carried out at anelevated temperature between about 1200° C. and 1400° C. However, it hasbeen found that temperatures as low as 750° C. are adequate to promotethe required reaction between the puffing inhibitor and coke particlesand can be employed.

The puffing inhibitor used in the process of the present invention maybe a salt of the alkali or alkaline earth metal, and preferably issodium carbonate. The inhibitor may be admixed with the petroleum cokeparticles before or after heating during the usual calcining process,and may be incorporated with the coke particles in the form of dry,granulated powders or as a solution containing the inhibitor which canbe sprayed onto the particles. The inhibitor is employed in amountsgreater than about 0.2 percent by weight of the coke.

In a preferred embodiment of the present invention, the improved processfor treating high sulfur petroleum coke particles comprises:

calcining the high sulfur petroleum coke particles;

adding sodium carbonate to the calcined coke particles at an elevatedtemperature above about 1200° C. but below the temperature at which thecoke particles would begin to puff in the absence of the sodiumcarbonate;

maintaining the calcined coke particles and sodium carbonate at theelevated temperature for a sufficient period of time to permit thesodium carbonate to react with the coke and to allow the resultingsodium to penetrate the particles and deposit sodium throughout the massof the particles; and

cooling the so-treated coke particles.

In another aspect of the present invention, a carbonaceous filler oraggregate is provided for use in the production of carbon or graphitearticles which comprises discrete particles of petroleum coke having ahigh sulfur content and having a puffing inhibiting agent distributedthroughout the mass of the particles. The puffing inhibiting agentcomprises a water-insoluble compound of an alkali or alkaline earthmetal selected from the group consisting of sodium, potassium, calciumand magnesium; the average amount of the metal in the particles beinggreater than about 0.15 percent by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic elevational view of a calcining apparatus whichhas been modified to carry out the process of the present invention;

FIG. 2 is an enlarged sectional view of the modified portion of theapparatus shown in FIG. 1;

FIG. 3 is a sectional view of the modified calcining apparatus takenalong the line 3--3 in FIG. 2;

FIG. 4 is a schematic elevational view of a calcining apparatusaccording to another embodiment of the present invention;

FIG. 5 is an enlarged side elevational view of the calcining apparatusshown in FIG. 4;

FIG. 6 is a graph showing the puffing rates of petroleum coke treatedwith a conventional inhibitor and the same coke treated according to thepresent invention;

FIGS. 7, 8 and 9 are graphs showing the puffing rates of severaldifferent types of petroleum cokes according to the present invention;

FIG. 10a is a photomicrograph taken with a Scanning Electron Microscope(SEM) at a magnification of 200X and showing an area near the edge of aninternal plane prepared by grinding a half-inch coke particle treatedaccording to the present invention;

FIG. 10b is a photomicrograph of the same area shown in FIG. 10a butshowing the sodium X-ray elemental map obtained by Energy DispersiveX-ray analysis (EDX), also at 200X magnification;

FIG. 10c is a photomicrograph of the EDX spectrum of the same area shownin FIGS. 10a and 10b;

FIG. 11a is a photomicrograph taken with a Scanning Electron Microscope(SEM) at a magnification of 45X and showing another area, closer to thecenter of the same internal plane shown in FIGS. 10a and 10b;

FIG. 11b is a photomicrograph of the same area shown in FIG. 11a butshowing the sodium X-ray elemental map obtained by EDX analysis, also at45X magnification;

FIG. 11c is a photograph of the EDX spectrum of the same area shown inFIGS. 11a and 11b;

FIG. 12a is a photomicrograph taken with a SEM at 50X magnification andshowing a third area of the same internal plane shown in FIG. 10a and10b;

FIG. 12b is a photomicrograph of the same area shown in FIG. 12a butshowing the sodium X-ray elemental map obtained by EDX analysis at thesame 50X magnification;

FIG. 12c is a photograph of the EDX spectrum of the same area shown inFIGS. 12a and 12b;

FIG. 13a is a photomicrograph taken with a SEM at 200X magnification andshowing a fourth area of the same internal plane shown in FIGS. 10a and10b;

FIG. 13b is a photomicrograph of the same area shown in FIGS. 13a butshowing the sodium X-ray elemental map obtained by EDX analysis at thesame 200X magnification;

FIG. 13c is a photograph of the EDX spectrum of the same area shown inFIGS. 12a and 12b;

FIG. 14a is a photomicrograph taken with a SEM at a 15X magnificationand showing both an internal plane prepared by grinding a quarter-inchcoke partical treated according to the present invention and alsoshowing an original pore surface exposed by grinding;

FIG. 14b is a photomicrograph of the same area shown in FIG. 14a butshowing the sodium X-ray elemental map obtained by EDX analysis at thesame 15X magnification;

FIG. 14c is a photograph of the EDX spectrum of the same area shown inFIGS. 14a and 14b;

FIG. 15a is a photomicrograph taken with a SEM at 15X magnification ofthe same surfaces shown in FIG. 14a but taken after the particle hadbeen leached with water;

FIG. 15b is a photomicrograph of the same areas shown in FIG. 14a butshowing the sodium X-ray elemental map obtained by EDX analysis at thesame 15X magnification;

FIG. 15c is a photograph of the EDX spectrum of the same areas shown inFIGS. 15a and 15b;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Petroleum coke is produced by coking heavy petroleum residues, as iswell known in the prior art. Raw petroleum coke, that is, petroleum cokethat has not been calcined, usually has a volatile matter content ofbetween about 6 and 14 percent. The volatile matter is typically removedby heating the raw petroleum coke in a calciner to temperatures ofbetween about 1200° C. and about 1400° C. Occasionally, calciningtemperatures as high as 1500° C. may be employed. The volatile mattercontent of a coke after calcination is usually less than about onepercent by weight. Raw petroleum coke is ordinarily reduced in size toparticles 4" or less prior to calcining.

For purposes of the present invention, the starting coke material may beeither a raw petroleum coke or a petroleum coke that has been calcinedby conventional methods. In either case, the petroleum cokes to whichthe present invention is particularly directed are the so called "highsulfur" petroleum cokes which ordinarily contain more than about 0.7percent by weight sulfur. These high sulfur petroleum cokes ordinarilycannot be adequately controlled by puffing inhibition methods that arepresently known in the art. Although these cokes cost less, their usefor production of carbon or graphite articles is either limited orrequires modified, more expensive processing technology.

Sulfur is released from its chemical bond with carbon when a petroleumcoke is heated to temperatures higher than about 1500° C., and in mostcases to at least about 1600° C., which is higher than ordinarycalcining temperatures. If this release of sulfur is not inhibited orthe sulfur is not tied up chemically inside the coke structure, then therapid escape of sulfur-containing vapors will create internal pressurein the coke particles which tends to expands the particles, sometimeseven splitting them or splitting the articles made therefrom. Thisphenomenon is called puffing.

It has been discovered in accordance with the present invention thatpuffing of the formed carbon or graphite article can be significantlyreduced or eliminated by treating the petroleum coke particles with analkali or alkaline earth metal compound and especially a salt of sodiumor potassium, such as sodium or potassium carbonate, at temperatureswhich are well below the temperature at which the coke begins to puff,prior to incorporating the coke particles into a carbonaceous mix. Fromthe literature, "Effect of Sodium Carbonate upon Gasification of Carbonand Production of Producer Gas," by D. A. Fox et al, Industrial andEngineering Chemistry, Vol. 23, No. 3, March 1931, it is known that analkali metal compound (e.g. sodium carbonate) can be effectively reducedwith carbon in a high-temperature reactor to produce alkali metal vaporsand carbon monoxide. It has been surprisingly found according to theinvention that if the alkali or alkaline earth metal compound is allowedto stand in contact with the petroleum coke particles for a sufficientlylong period of time, e.g. about one minute or more, while maintainingthe temperature above that at which this reduction reaction occurs, e.g.about 750° C. in the case of sodium carbonate, then the alkali oralkaline earth metal, so produced, will penetrate and form an alkali oralkaline earth metal containing deposit throughout the mass of the cokeparticles not just into their pores. A residence time of 30 seconds hasbeen shown in the laboratory to be effective for suppression of puffing.In production scale trials the residence time at the reactiontemperature was maintained longer than one minute.

It has been known for some time that sodium carbonate, when used as aninhibitor in the conventional way, adding to the coke-pitch mix, causesthe product to have a lower density and a lower strength compared to thesame product made with the conventional puffing inhibitor; i.e. ironoxide. We found that sodium carbonate, when used as a puffing inhibitorin accordance with this invention, did not cause a loss of eitherdensity or strength in the product and yielded a product equal to thatproduced using iron oxide as the puffing inhibitor.

Since the inhibiting agent is deposited inside the coke particle, it hasno contact with the pitch during processing of the carbonaceous mix anddoes not interfere with any extrusion aids such as fatty acids.

Although the alkali or alkaline earth metal compound may be placed incontact with the petroleum coke particles either before or after heatingthe coke particles to the required temperatures for carrying out thereaction, it is highly advantageous to add the inhibitor compound to thecoke particles in the form of dry, granulated powder after the cokeparticles have been heated to calcining temperatures between about 1200°C. and about 1400° C. In actual practice, the dry, granulated powder ofinhibitor compound is added to the calcined coke particles at thedischarge end of the calciner. It is also possible to add the inhibitorcompound to the raw coke in the form of dry powder or to spray the cokewith a solution or slurry containing the inhibitor prior to calcination.

The alkali or alkaline earth metal compound, e.g. sodium carbonate, isadmixed with the petroleum coke particles in amounts greater than about0.2 percent by weight. Preferably, the inhibitor is used in amountsranging from about 0.5 to about 2.5 percent by weight of the coke.

In FIGS. 1-3 of the drawing, there is shown a typical rotary typecalcining apparatus which has been modified in order to carry out theimproved process of the present invention. As shown, the calciningapparatus includes an elongated, cylindrical, rotary calcining kiln 10having an inlet end 12 and an outlet end 14. The inlet end 12 of thecalcining kiln 10 is mounted for rotation within a stationary cokeentrance chamber 16 having a vertical stack or chimney 18 for the escapeof flue gases from inside the calciner. The outlet end 14 of thecalcining kiln 10 is similarly mounted for rotation within a stationarycoke discharge chamber 20 including a conventional clinker box 22disposed vertically below the chamber 20.

Raw petroleum coke particles 24 are supplied to the calcining apparatusvia a horizontal conveyor 26 and are fed down a coke chute 28 into theinlet end 12 of the rotary calcining kiln 10. As shown in the drawing,the kiln 10 is inclined at a small angle along its longitudinal axisfrom its inlet end 12 down to its outlet end 14. Thus, as the cokeparticles 24 enter the kiln 10, they are forced by gravity to moveslowly along the length of the kiln 10 as it rotates until they reachthe outlet end 14 from whence they are discharged to the chamber 20.

A fuel, such as natural gas, is burned at the hot end of the kiln andthe combustion gas passes through the kiln 10 counter-currently to theflow of coke particles 24. The hot combustion gases heat the cokeparticles 24 and cause the volatiles contained therein to vaporize andburn.

The hot calcined coke particles 24 drop from the chamber 20 into theclinker box 22 where they flow over the refractory block 30 (FIG. 2).The block 30 is located in the bottom of a rectangular outlet opening 32provided in the stationary head 34 of the cooler 36.

An elongated, cylindrical, rotary cooler 36 is positioned beneath thedischarge chamber 20. The cooler 36 has an inlet end 38 which is mountedfor rotation around the stationary head 34 of the clinker box 22. Theoutlet end 40 of the cooler 36 is mounted for rotation within astationary coke delivery chamber 42.

The elongated, cylindrical, cooler 36 is also inclined downwardly at aslight angle from its inlet end 38 to its outlet end 40. As shown inFIG. 2, the hot calcined coke particles 24 collect in a body at thebottom of the clinker box 22 behind the refractory block 30 andeventually spill over the edge of the block 30 and fall into the inletend 38 of the rotary cooler 36. The coke particles are then forced bygravity and rotation of the cooler to move slowly down the length of thecooler 36 until they reach the outlet end 40 from whence the particlesentter and collect within the coke supply chamber 42.

Although some calciners may employ indirect cooling, e.g. through thesteel shell of the cooler 36, most calciners quench the hot, calcinedcoke directly by spraying it with water. This direct spraying reducesthe temperature of the hot coke particles immediately after they leavethe clinker box 22. Typically, in order to accomplish this purpose, aseries of nozzles are provided just below the outlet opening 32 of theclinker box 22.

As shown in FIG. 2, a conventional calcining apparatus can be modifiedto carry out the process of the present invention by incorporating a hotzone 44 inside the inlet end 38 of the cooler 36. The hot zone is formedin accordance with the present invention by locating a circulrrefractory ring 46 a predetermined distance down stream from the clinkerbox outlet 32 and by moving the quench-water spray nezzles 56 downstreamof the refractory ring 46. As shown, the ring 46 is mounted against therefractory lining 45 which is placed adjacent to the interiorcylindrical side walls of the cooler 36. The refractory retention ring46 increases the depth of the coke layer in the hot zone 44 and therebyincreases the coke residence time. The temperature of the coke particles24 as they enter the hot zone 44 is somewhat reduced by the processreaction but remains above 1100° C.

Dry, granulated powder 48 of sodium carbonate is fed into the hot zone44 through a funnel 50. The funnel 50 has an elongated, tubular stem 52which extends through the side wall 34 of clinker box 22 and depositsthe powder on top of the layer of hot calcined coke particles 24 at thebottom of the hot zone 44. As best shown in FIG. 3, the powder is mixedwith the coke particles 24 by the tumbling action occurring inside therotating cooler 36. The powdered sodium carbonate melts upon contactwith the hot coke particles 24 and reacts with the coke according to thefollowing endothermic reaction:

    Na.sub.2 CO.sub.3 (l)+2C(s)=2Na(g)+3CO(g)

    ΔH=213 kcal/mol - - - at 1330° C.

(l), (s), and (g) refer to the physical state of the reactants, i.e.liquid, solid and gaseous, respectively. The elemental sodium producedby the above reaction penetrates the coke particles and is distributedthroughout the mass of the coke particles creating a modified cokecontaining sulfur and sodium.

After treatment with the sodium carbonate powders in the hot zone 44 fora sufficient period of time the hot calcined coke particles 24eventually flow over the refractory ring 46 and into the cooling section53 of the cooler 36.

In this modified version of the cooler 36, a pipe 54 carrying quenchingwater to a series of nozzles 56 at its outer end, is mounted in theusual manner within the lower portion of the side wall 34 of clinker box22 but in this case the pipe 54 is made longer so as to extendcompletely through the hot zone 44 and into the cooling section 53. Thusthe water is sprayed from the nozzles 56 directly onto the hot cokeparticles as they leave the hot zone 44 to quench the particles andsignificantly reduce their temperature.

The quenched or cooled, treated, calcined coke particles are thendischarged from the chamber 42 onto a moving conveyor 58 whichtransports the coke particles to a storage area. Steam, produced in thecooler from the quenching water, is removed from the cooler togetherwith some air by a fan 62 and blown to atmosphere. The steam/air mixturepasses through a dust collector 60 where coke dust is trapped to preventair pollution.

FIGS. 4 and 5 show a calcining apparatus which is constructedspecifically for use in treating petroleum coke according to the presentinvention. This calcining apparatus is equipped with a retention chambercomprising a separate reactor vessel 68. This reactor vessel is locateddownstream from the calciner and upstream from the cooler and can bedesigned for a long residence time. Calcined coke particles are fed fromthe discharge chamber 20 to the reactor vessel 68 where they are treatedwith dry, granular powders of the alkali or alkaline earth metalcompound, e.g., sodium carbonate, which is supplied simultaneouslythrough the inlet 70. After treatment, the hot coke particles pass outthrough the outlet 72 in reactor vessel 68 and enter the inlet end 38 ofthe rotary cooler 36.

It will be seen from the foregoing that the process of the presentinvention can be practiced either in an existing facility using aconventional calcining apparatus or in a new facility employing acalcining apparatus provided with a separate reactor according to thepresent invention.

An important advantage which is obtained by adding the inhibitor e.g.sodium carbonate, to the calcined petroleum coke particles in a separatereaction vessel located at the discharge end of the calcining kiln isthat no gas flows through this vessel and hence there is virtually noopportunity for the inhibitor to be carried away and released to theatmosphere.

A number of laboratory experiments were conducted to determine theamount of sodium carbonate required in the present process for effectivesuppression of puffing and also the minimum residence time in the caseof four different petroleum cokes having different sulfur contents. Inthese experiments, one kilogram of calcined coke particles was placedinto an open-top graphite container and inserted into a muffle furnacepreheated to about 1200° C. When the coke temperature (measured by athermocouple in the coke) reached 1200° C., the furnace door was openedand a predetermined amount of sodium carbonate, e.g. 0.4%, 0.8%, 1.2%,1.6%, etc., was dropped on the coke surface using a long graphite tool.The coke sample was then raked briefly. At a predetermined time, thegraphite container was pulled out of the furnace and the coke quenchedby spraying water on it and raking it at the same time. The timerequired to reduce the coke temperature to between 300° C. and 500° C.ranged from about 30 seconds to about 90 seconds.

The experimental reaction time reported was counted from the moment ofdropping the inhibitor onto the coke to the moment when thewater-quenching was started. The quenched coke was allowed to cool toambient temperature without further water spraying. The cooled cokesamples were then tested for puffing, i.e., the irreversible expansionoccurring in sulfur-containing cokes when heated to between about 1600°C. and 2200° C.

Puffing was measured on a specimen prepared from the coke and placed ina dilatometer assembly made from a low-expansion graphite. The assembly,containing the specimen, was placed in a tube furnace and heated at 450°C. per hour to 2400° C. After the temperature had reached 1000° C., thedifferential expansion of the specimen over that of the graphitecontainer was recorded at 15 minute intervals.

Several different values can be derived from these measurement, i.e.,(1) the total expansion over the temperature range; (2) the puffing rateper unit of time as a function of temperature; and (3) the temperatureat which the puffing rate reaches a maximum.

FIGS. 6 through 9 show relationships between the highest puffing grateand the amount of inhibitor used. The unit of puffing rate in thosefigures is 10⁻⁴ m/m per 15 minutes at a heating rate of 450° C. perhour. The temperature at which the puffing rate of these particularcokes attained its highest value was at about 1750° C.

FIG. 6 is a graph showing the relationship between the maximum puffingrate as determined in the above experiment and the amount of inhibitorused. Curve A shows this relationship in the case of a needle coke, cokeD¹, containing 1.05 percent by weight sulfur and using different amountsof sodium carbonate as the inhibitor. A puffing rate of about ten is thedesired limit for processing the coke into graphite electrodes by moderngraphitization methods. It will be seen from Curve A that thispermissible puffing rate is achieved with only one percent by weight ofthe sodium carbonate inhibitor.

For purposes of comparison, the same experiment described above wasrepeated with the same needle coke having the same sulfur content butusing a conventional inhibitor, iron oxide. Curve B in FIG. 6 shows theresults of this experiment. It will be seen that the puffing suppressionin the case of the conventional inhibitor was far inferior to thatobtained with the same coke treated with sodium carbonate according tothe present invention. The iron oxide, even when used at twice theconventional concentration (4 weight percent instead of 2 weightpercent), did not attain a comparable reduction in the puffing of thisparticular coke.

The same type of experimental test was conducted on a regular gradepetroleum coke, coke E¹, containing 1.3 percent by weight sulfur. Inthis test, the coke was treated according to the process of the presentinvention using sodium carbonate as the inhibitor and a residence timeof about one minute. The results of this test are represented by thecurve in FIG. 7. It will be seen that an adequate puffing rate reductionis achieved when using only about 0.6 weight percent of the sodiumcarbonate inhibitor.

A similar experimental test was conducted on another calcined petroleumneedle coke, coke F¹, containing about 1.3 weight percent sulfur usingsodium carbonate as the inhibitor and a residence time of about oneminute. The results of this test are represented by the curve in FIG. 8.It will be seen that this particular coke required about 1.3 weightpercent of the sodium carbonate inhibitor in order to suppress puffingbelow the permissible level.

Another experimental test was conducted on another needle coke, coke G¹,contaning 1.1 weight percent sulfur again using sodium carbonate as theinhibitor and a residence time of about one minute. The results of thistest are represented by the curve in FIG. 9. It will be seen that inthis case about 1.2 weight percent of the sodium carbonate inhibitor wasrequired in order to suppress the puffing below the permissible puffingrate. The same type of coke, coke G¹, required about 1.6 weight percentof the sodium carbonate inhibitor when its sulfur content increased toabout 1.25 weight percent sulfur.

A number of large scale experimental trials have also been conductedusing a modified calcining apparatus as substantially shown in FIGS. 1-3wherein several hundred tons of three different regular and needle cokescontaining about one weight percent or more of sulfur were calcined andtreated according to the process of the present invention. In thesetrials, approximately one weight percent of sodium carbonate powder of asize smaller than 800 microns wase added to the calcined coke in a hotzone constructed inside the inlet end of the cooling drum while attemperatures of between 1200° C. and 1350° C. and for a period of atleast one minute. The calcined and treated coke was then cooled andsamples were taken and subjected to the same type of test as describedabove to determine the puffing rate. It was found that puffing of theseparticular cokes had been reduced sufficiently for rapid lengthwisegraphitization. It was also unexpectedly found that the present processreduced substantially the amount of chemicals, e.g., chlorides,sulfates, etc., that are normally released to the atmosphere in thecooler off-gas during calcination. Moreover, since the process alsoeliminates the acidity of the cooler off-gas, the potential forequipment corrosion is substantially reduced.

Graphite electric furnace electrodes measuring 20 inches in diameter and96 inches in length were made using one of the high sulfur petroleumneedle cokes calcined and treated in the above described experimentaltrials. The calcined and treated coke was used as an aggregate or fillerand mixed with a pitch binder and the usual extrusion aids to form acarbonaceous mix. The mix was then extruded, baked at about 800° C. andthen graphitized to temperatures of about 3000° C. There were noprocessing problems during extrusion and baking and there was noevidence of any puffing problems. The electrodes were subsequentlytested experimentally on an electric-arc steel furnace and performedcomparably to electrodes made from more expensive, low-puffing premiumneedle cokes.

Particles of a regular grade coke, coke E¹, containing an average 1.28percent sulfur, were treated in accordance with this invention withvarying proportions of sodium carbonate ranging from 0.25 percent to 1percent. The treated particles were then tested, using routineanalytical methods, for contents of sulfur, sodium, and ash, and weretested for puffing. The resulting are assembled in Table 1. The datashows (1) that addition of 0.55% sodium carbonate reduced the puffing ofthis coke to an acceptable level, while 0.25% did not; (2) that thesodium content in the coke was proportional to the amount of sodiumcarbonate added during the treatment within experimental error, and (3)that 0.18% sodium content, corresponding to 0.55% of Na₂ CO₃ added,reduced the puffing of this particular coke to an acceptable level,while 0.12% sodium in the coke was not sufficient.

                  TABLE I                                                         ______________________________________                                        Sample   % Na.sub.2 CO.sub.3                                                                     Puffing   % Ash  % Na                                      No.      Added     Rate      in Coke                                                                              in Coke                                   ______________________________________                                        Control  0         62.0                                                       1        1         0         1.88   0.36                                      2        0.85      2.3       1.22   0.26                                      3        0.7       8.7       1.0    0.24                                      4        0.55      11.3      0.76   0.18                                      5        0.25      41.0      0.68   0.12                                      ______________________________________                                    

Penetration of sodium into the body of the particles, treated inaccordance with this invention, was examined by a Scanning ElectronMicroscope using an Energy-Dispersive X-ray Method (SEM-EDX). Theparticles were mounted in epoxy and ground to mid-level to expose aninternal plane and also leave a natural pore surface.

In FIGS. 10a-13a, inclusive, there are shown a series ofphotomicrographs taken at different magnifications (i.e., 200X, 45X, 50Xand 200X, respectively), and showing SEM images of three areas of aninternal plane produced by grinding a quarter-inch coke particle. Thearea shown in FIG. 10a is near the edge of the internal plane, the areashown in FIG. 11a is close to the center of the plane, and the areashown in FIG. 12a is in the center of the ground plane. The fourth areashown in FIG. 13a is also close to the center of the plane, similar tothe area shown in FIG. 11a.

The location and distribution of sodium at the internal plane is shownin the photomicrographs in FIGS. 10b-13b, inclusive. Thephotomicrographs were produced at the same magnifications indicatedabove by EDX analysis for sodium using a Scanning Electron Microscope.

It will be seen from the fairly uniform distribution of bright dotsthroughout the photomicrographs, each of which represents a differentarea in the same internal plane of the coke particle, that sodium doesin fact penetrate deep inside each particle treated according to theprocess of the present invention and that the distribution of sodiumthroughout the mass of each individual coke particle is substantiallyuniform. The concentration of sodium may vary from one particle toanother but inside of an individual particle, the concentration isessentially uniform. It should be understood that the sodium produced bythe reaction between sodium carbonate and coke forms, after diffusioninto the mass of the coke particles, a compound that is not soluble inwater and is not reactive with water, and that the sodium is present asa sodium containing compound rather than as elemental sodium. The exactcomposition of the sodium containing compound is not clearly understoodat this time.

A series of energy spectrum charts taken at the ground internal surfacesof each zone of the coke particles examined in these tests are shown inFIGS. 10c-13c, inclusive. It will be seen from the charts that theintensity of two peaks predominate in the energy spectrum and that thesepeaks are located at the same two positions corresponding to both sodiumand sulfur, thus confirming the presence of these two elements in thecoke particles. Moreover, since a peak for sodium occurs in each chartrepresenting a different zone of the coke particle, it can be concludedthat sodium is actually deposited substantially uniformly throughout themass or body of the coke particles treated according to the presentinvention.

Still another study of sodium penetration and of its solubility afterthe reaction with coke has been carried out with particles of Coke F¹,0.12 inches to 0.25 inches in size, which were treated with 20 percentsodium carbonate at about 1200° C. in accordance with the presentinvention. One of these treated particles was mounted and ground toexpose both an internal plane and an original pore surface. Thisparticle was examined with the same SEM-EDX methods as the particleshown in FIGS. 10a through 13a. After the examination, the particle wasleached with water to remove any water soluble compounds, and then itwas again examined using the same techniques. FIGS. 14a, 14b and 14cshow the examinations before leaching, while FIGS. 15a, 15b and 15c showthe examinations after leaching. FIG. 14b demonstrates that the sodiumwas distributed essentialy uniformly at the ground internal plane andalso substantially uniformly, but at a much higher concentration, on theexposed original surface of the pore. FIG. 15b shows that afterleaching, the penetration and distribution of the sodium at the internalplane remained essentially unchanged, but the sodium concentration onthe original pore surface was reduced to approximately the same level ason the internal plane and its distribution was essentially uniform.

It is believed that the insoluble sodium, observed in the above study,is the product of the interaction between sodium and coke,while thewater-soluble sodium, found only on the original surface but not insidethe body of the particle, is unreacted sodium carbonate.

Analyses of the water-extract by standard analytical methods confirmedthe presence of sodium carbonate. The presence of unreacted sodiumcarbonate on the surface of the treated particles indicates that, undersome reaction conditions, the reaction between sodium carbonate and cokedid not proceed to completion.

Thus, the present invention provides an improved method for treatingcalcined petroleum coke in order to reduce or eliminate puffing whereinthe coke particles are heated in the presence of an alkali or alkalineearth metal compound, preferably sodium carbonate, at temperatures ofabove about 750° C. and preferably between about 1200° C. and 1400° C.The inhibitor should be maintained in contact with the coke particlesfor a sufficiently long period of time e.g., one minute or more, toallow the inhibitor to react with the carbon and to allow products ofthe reaction to penetrate deeply into the mass of the coke particles.Although it is possible to add the inhibitor directly to the raw cokeprior to heating or calcining, it is preferred to add the inhibitorimmediately after the coke particles have been discharged from thecalciner. This avoids possible environmental problems and also has theadvantage of reducing the off-gas acidity, as explained hereinabove.

The present invention further provides an improved method for producingcarbon and graphite particles such as electric furnace electrodeswherein the treated coke is incorporated with a conventional pitchbinder to form a carbonaceous mix which is then shaped or extruded,baked to carbonize the binder and, if desired, graphitized. Theprincipal advantage offered by this improved process is that themanufacturer of carbon and graphite articles or electrodes can nowemploy lower-priced, high sulfur petroleum cokes and yet producehigh-quality electrodes.

What is claimed is:
 1. A process for treating high sulfur petroleum cokeparticles which comprises:reacting said petroleum coke particles in theabsence of a binder with a compound containing an alkali metal selectedfrom the group consisting of sodium and potassium at an elevatedtemperature above that at which said compound begins to react withcarbon but below the temperature at which said coke particles wouldbegin to puff in the absence of said compound; maintaining said cokeparticles and said compound at said elevated temperature for asufficient period of time to permit the reaction to proceed and to allowproducts of the reaction to penetrate said coke particles and form asodium or potassium-containing deposit throughout the mass of saidparticles; and cooling the so-treated coke particles.
 2. A processaccording to claim 1, wherein said coke particles are raw petroleumcoke.
 3. A process according to claim 1, wherein said coke particles arecalcined coke.
 4. A process according to claim 1, wherein said alkalimetal compound is admixed with said coke particles prior to reactingsaid alkali metal compound with said coke particles at said elevatedtemperature.
 5. A process according to claim 1, wherein an aqueoussolution containing said alkali metal compound is sprayed onto said cokeparticles prior to reacting said alkali metal compound and said cokeparticles at said elevated temperature.
 6. A process according to claim1, wherein said coke particles are heated to a temperature of at leastabout 750° C. prior to reacting said coke particles with said alkalimetal compound at said elevated temperature.
 7. A process according toclaim 1, wherein said elevated temperature is between about 1200° C. and1400° C. and wherein said alkali metal compound is in the form of dry,granulated powders.
 8. A process according to claim 1, wherein saidalkali metal compound is sodium carbonate.
 9. A process according toclaim 1, wherein said alkali metal compound is potassium carbonate. 10.A process for treating high sulfur petroleum coke particles whichcomprises:calcining said petroleum coke particles; adding sodiumcarbonate to the calcined coke particles at a temperature above about1200° C. but below the temperature at which said calcined coke particlesbegin to puff in the absence of said sodium carbonate; maintaining saidcalcined coke particles and said sodium carbonate at said temperaturefor a sufficient period of time to permit said sodium carbonate to reactwith carbon and to allow products of the reaction to penetrate said cokeparticles and form a sodium containing deposit throughout the mass ofsaid calcined coke particles; and cooling the so-treated coke particles.11. A process according to claim 10, wherein said sodium carbonate isadded to said calcined coke particles in the form of dry granulatedpowders.
 12. A process according to claim 10, wherein said sodiumcarbonate powders are added to said calcined coke particles at atemperature within a range of above about 1200° C. to about 1400° C. 13.A process according to claim 12, wherein said sodium carbonate powdersare added to said calcined coke particles in amounts greater than about0.2 percent by weight of the to calcined coke particles.
 14. A processaccording to claim 11, wherein said calcined coke particles and saidsodium carbonate powders are maintained at said temperature for at leastabout 30 seconds.
 15. In a process for calcining a raw petroleum cokewherein particles of said coke are passed through a rotating calcinerhaving a discharge end while heating said particles to calciningtemperatures;the improvement whereby said calcined coke particles aretreated to inhibit puffing of said calcined coke, said improvementcomprising: adding sodium carbonate in the form of dry, granulatedpowder to the calcined coke particles in a hot zone communicating withsaid discharge end of said calcining kiln at a temperature within therange of between about 1200° C. and 1400° C.; and maintaining saidcalcined coke particles and sodium carbonate at said temperature whilein said hot zone for a sufficient period of time to permit said sodiumcarbonate to react with carbon and to allow products of the reaction topenetrate said calcined coke particles and form a sodium containingdeposit throughout the mass of said calcined coke particles.
 16. Aprocess according to claim 15, wherein said sodium carbonate powder isadded to said calcined coke particles in amounts ranging from about 0.2to about 2.5 percent by weight of the calcined coke particles.
 17. Aprocess according to claim 16, wherein said sodium carbonate powder isadded to said calcined coke particles in amounts ranging from about 0.5to about 2.5 percent by weight of the calcined coke particles.
 18. Aprocess according to claim 15, wherein said calcined coke particles andsaid sodium carbonate are maintained at said temperature for at leastabout one minute.
 19. A process for producing a carbon article from ahigh sulfur petroleum coke comprising:reacting particles of saidpetroleum coke with a compound containing an alkali metal selected fromthe group consisting of sodium and potassium at an elevated temperatureabove that at which said compound begins to react with carbon but belowthe temperature at which said coke particles would begin to puff in theabsence of said compound; maintaining said coke particles and saidcompound at said elevated temperature for a sufficient period of time topermit the reaction to proceed and to allow products of the reaction topenetrate said coke particles and form a sodium or potassium-containingdeposit throughout the mass of said particles; cooling the so-treatedcoke particles; mixing the cooled coke particles with a pitch binder;forming the mixture into an article of desired shape; and baking theshaped article to a temperature sufficient to carbonize the binder. 20.A process according to claim 19, wherein the baked article is heated toa sufficiently high temperature to graphitize said article.
 21. Aprocess according to claim 19, wherein said elevated temperature isbetween about 1200° C. and 1400° C. and wherein said alkali metalcompound is in the form of dry, granulated powders.
 22. A process forproducing a carbon electrode from a high sulfur petroleum cokecomprising, in combination:calcining particles of said petroleum coke;adding sodium carbonate in the form of dry, granulated powder to thecalcined coke ptartcles at a temperature above about 1200° C. but belowthe temperature at which said calcined coke particles begin to puff inthe absence of said sodium carbonate; maintaining said calcined cokeparticles and said sodium carbonate at said temperature for a sufficientperiod of time to permit sodium carbonate to react with carbon and toallow products of the reaction to penetrate said calcined coke particlesand form a sodium containing deposit throughout the mass of saidcalcined coke particles; cooling the so-treated calcined coke particles;mixing the cooled calcined coke particles with a pitch binder; formingthe mixture into the desired shape of said electrode; and baking theshaped electrode to an elevated temperature sufficient to carbonize thebinder.
 23. A process according to claim 22, wherein said shapedelectrode is heated to elevated temperature above about 2800° C. tographitize said electrode.